Bases of Igneous Rock Classification

• Mineral Content:
  • This classification relays on minerals present in the rock (the mode).
  • Principal minerals: Silicate minerals (Bowen’s Series).
  • Felsic minerals (quartz, plagioclase, alkali feldspar, muscovite).
  • Mafic minerals (biotite, hornblende, pyroxene, olivine).
• Chemical Composition:
  • Silicic (SiO₂), Alkaline (Na₂O + K₂O), Aluminous (Al₂O₃).
  • Acidic (> 66% SiO₂), Intermediate (52–66% SiO₂), Basic (45–52% SiO₂), Ultrabasic (< 45% SiO₂).
• Color:
  • Leucocratic: Light-colored rocks.
  • Melanocratic: Dark-colored rocks.
  • Color index (M): Percentage of dark minerals.

Issues with Terminology

• Felsic/Mafic:
  • Terms are based on mineral composition but often used based on color.
  • Example: Dark-colored quartz and feldspars are still felsic, leading to confusion.
• Acidic/Basic:
  • Terms derived from outdated concepts (silicic acid in solution).
  • Not accurate for describing magmas or modern petrology.
  • Basic rocks are generally mafic, but chemical content should take precedence over color.

Terms in Literature

• Mafic and Felsic:
  • Applied to both rocks and magmas.
  • Considered appropriate despite being mineral-based.
• Basic and Acidic:
  • Applied to magmas and based on chemical composition rather than mineralogy.
• Other terms:
  • Descriptive terms like “basaltic magma” and “andesitic magma” name the magma based on its solidified equivalent.
  • Use of precise chemical descriptions (e.g., “silicic” or “alkaline”) for unusual concentrations of certain components.
• Terminological inconsistencies can lead to confusion in the classification and description of igneous rocks and magmas.
• While terms like “felsic” and “mafic” dominate North American literature, British geologists prefer “basic” and “acidic,” focusing more on chemical composition.
• A balance between mineral-based and chemical-based descriptions is essential for clarity in igneous rock classification.

The IUGS Classification

• This Classification was developed by the International Union of Geological Sciences (IUGS) to standardize igneous rock nomenclature.

• The IUGS system requires that we determine the mineral components of a rock and plot the percentages of three of those components on appropriate triangular diagrams to determine the proper name.

Plotting on a Triangular Diagram

Steps to Plot 70% X, 20% Y, and 10% Z

1. Understand the Axes of the Triangular Diagram

• Each apex represents 100% of a component:
  • X at the top apex.
  • Y at the bottom right.
  • Z at the bottom left.
• Percentages decrease from each apex to the base opposite the apex.
  • The sum of X, Y, and Z must equal 100%.

2. Plot the 70% X Value

• Find the X-axis:
  • 70% X is represented by counting 7 divisions (70%) from the Y–Z base toward the X apex.

3. Plot the 20% Y Value

• Move along the Y-Z base:
  • Starting from the Z apex, move 67% of the way toward Y (because Y/(Y + Z) = 20/30 = 67%).

4. Locate the Point

• Draw a line from the 67% Y-Z point toward the X apex.
• Mark the point where this line intersects the 70% X line.

5. Verify the Z Value

• The point should also align with 10% Z by counting up 1 division from the X–Y base toward the Z apex.
• The intersection of these lines represents 70% X, 20% Y, and 10% Z.

Final Result

• The point A representing 70% X, 20% Y, and 10% Z is plotted where the 70% X line, the 10% Z line, and the 20% Y line intersect.

Classification Based on Mineral Content

• For most igneous rocks: Q + P + A (Quartz, Plagioclase, Alkali Feldspar).
• If feldspathoids are present: F + P + A (Feldspathoids, Plagioclase, Alkali Feldspar).

Classification of The Plutonic Igneous Rocks

Classification of Volcanic Rocks

Chemical Classification of Volcanic Rocks

Chemical Classification

1. Silica Saturation
2. Alumina (Al2O3) Saturation
3. Alkaline/Subalkaline Rocks

Silica Saturation

If a magma is oversaturated with respect to silica, then a silica mineral such as quartz, cristobalite, tridymite, or coesite should precipitate from the magma and be present in the rock. On the other hand, if a magma is undersaturated with respect to silica, a silica mineral will not precipitate and thus will not be present in the rock. This concept divides rocks into three categories:

1. Silica Undersaturated Rocks

These rocks contain minerals that, in general, do not coexist with quartz. Such minerals include:

• Nepheline (NaAlSiO₄)
• Leucite (KAlSi₂O₆)
• Forsteritic Olivine (Mg₂SiO₄)
• Sodalite (3NaAlSiO₄ · NaCl)
• Nosean (6NaAlSiO₄ · Na₂SO₄)
• Haüyne (6NaAlSiO₄ · (Na₂,Ca)SO₄)
• Perovskite (CaTiO₃)
• Melanite (Ca₂Fe³⁺Si₃O₁₂)
• Melilite ((Ca,Na)₂(Mg,Fe²⁺,Al,Si)₃O₇)

These minerals indicate a silica-undersaturated rock. When calculating a CIPW norm, silica-undersaturated rocks will contain normative nepheline and/or leucite.

Note

The CIPW Norm (Cross, Iddings, Pirsson, Washington Norm) is a method used to calculate the idealized mineral composition of an igneous rock based on its chemical analysis. It provides a way to standardize and classify rocks by computing a theoretical set of minerals (the norm) that would form from a complete crystallization of the rock’s magma under dry conditions at low pressure.

2. Silica Oversaturated Rocks

These rocks can be identified as those that do not contain any of the minerals listed above. When calculated in the CIPW norm, silica-oversaturated rocks will contain normative quartz.

3. Silica Saturated Rocks

Silica-saturated rocks contain just enough silica that neither quartz nor the silica-undersaturated minerals appear. In the CIPW norm, these rocks typically contain olivine or hypersthene + olivine, but lack quartz, nepheline, and leucite.

Alumina (Al₂O₃) Saturation

1. Peraluminous Rocks

• Condition: Alumina (Al₂O₃) exceeds the amount required to form feldspars.
• Chemical Expression: Al₂O₃ > (CaO + Na₂O + K₂O)
• Expected Minerals: Al₂O₃-rich minerals such as muscovite, corundum, topaz, kyanite, andalusite, or sillimanite.
• CIPW Norm: Contains corundum (Al₂O₃) and lacks diopside.

2. Metaluminous Rocks

• Condition: Alumina is balanced with alkali and alkaline earth oxides.
• Chemical Expression: Al₂O₃ < (CaO + Na₂O + K₂O) and Al₂O₃ > (Na₂O + K₂O)
• Expected Minerals: Lack Al₂O₃-rich minerals, sodic pyroxenes, and amphiboles.
• Common Examples: Most common igneous rocks like diorite, gabbro.

3. Peralkaline Rocks

• Condition: Excess of alkalies (Na₂O + K₂O), undersaturated in alumina.
• Chemical Expression: Al₂O₃ < (Na₂O + K₂O)
• Expected Minerals: Na-rich minerals like aegerine, riebeckite, arfvedsonite, or aenigmatite.
• CIPW Norm: Contains acmite (NaFe³⁺Si₂O₆) and/or sodium metasilicate (Na₂SiO₃).

Alkaline vs Subalkaline Rocks

Rocks can be classified as alkaline or subalkaline based on their alkali content relative to silica, using an alkali vs. silica diagram.

1. Alkaline Rocks

• Characteristics: High in alkali (Na₂O + K₂O) compared to silica.
• Silica Saturation: Often silica undersaturated, but not always peralkaline.
• Relation to Peralkaline: While peralkaline rocks are generally alkaline, not all alkaline rocks are peralkaline.

2. Subalkaline Rocks

• Characteristics: Lower in alkali content relative to silica.
• Silica Saturation: Tend to be more silica-saturated.

Note: Very alkaline rocks, which plot well above the dividing line in the alkali vs. silica diagram, are usually silica undersaturated.

Classification of Pyroclastic Rocks

• Pyroclastic Rocks: Formed from volcanic fragments (pyroclasts) during explosive eruptions.
• Classified based on:
  • Type of fragmental material.
  • Size of the fragments.
  • Chemical or modal composition (if possible, when impurities are minimal).

Fragment Size Classification

• > 64 mm: Bombs (if molten during fragmentation—thus typically rounded/blobby, flattened, or stretched) or Blocks (if not molten during fragmentation—thus typically angular or broken).
• 2–64 mm: Lapilli.
• < 2 mm: Ash.

Key IUGS Requirements

• A true pyroclastic rock must contain at least 75% pyroclasts.
  • Pyroclasts can include individual crystals, glass, or rock fragments.
  • Crystals are referred to as crystal fragments, not phenocrysts.

Types of Pyroclastic Rocks

• Tuffs: Classified based on fragment type.
  • Coarse Ash Tuff: Fragments 1/16 mm to 2 mm.
  • Fine Ash Tuff (Dust Tuff): Fragments < 1/16 mm.
  • Lithic Tuff: Dominated by rock fragments.
  • Vitric Tuff: Dominated by pumice/glass fragments.
  • Crystal Tuff: Dominated by crystal fragments.
• Example: Rhyolitic Lapilli Tuff - A pyroclastic rock dominated by lapilli-sized fragments with a rhyolitic composition.

Mixed Pyroclastic and Sedimentary Rocks

• Tuffite: Contains both pyroclastic and sedimentary clastic material (epiclasts).
  • Can be further classified (e.g., Tuffaceous Sandstone).
• Epivolcaniclastics: Secondary deposits, not directly from eruptions (e.g., volcanic mudflows or reworked epiclasts).

Special Types of Pyroclastic Deposits

• Aquagene Tuff: Waterborne ash accumulation, either from subaqueous eruptions or reworked by water.
• Hyalotuff/Hyaloclastite: Created when magma is shattered upon contact with water.

Other Ignous Rocks